Environment

 10 in1-smtp.messagingengine.com
 20 in2-smtp.messagingengine.com

 autoconfig.<domain>   → autoconfig.fastmail.com
 autodiscover.<domain> → autoconfig.fastmail.com

 v=spf1 include:spf.messagingengine.com ?all

 fm1/fm2/fm3 selectors pointing to *.dkim.fmhosted.com

 v=DMARC1; p=reject; adkim=r; aspf=r; rua=...

  ```
  Authentication error.
  Component returned failure code: 0x80550014 [nsIMsgIncomingServer.verifyLogon]
  ```

 * Cleared Saved Passwords.
 * Removed/re-added accounts.
 * Used Advanced Config to hard-set `imap.fastmail.com:993 SSL/TLS` and `smtp.fastmail.com:465 SSL/TLS`. Thunderbird still overwrites SMTP on test.

 * Deleted `~/Library/Thunderbird/` (profiles, installs.ini, profiles.ini).
 * Cleared Keychain items for imap/smtp/domain.
 * Flushed macOS DNS cache.

  * Is there an about\:config pref to disable the probe/replace behavior during SMTP test?
  * Can Thunderbird be told to prioritize domain autoconfig and skip ISPDB/MX guessing entirely?

  * Beyond deleting `~/Library/Thunderbird` and Keychain, are there other places Thunderbird stores provider data?

  * Which logging prefs/console settings show:

    * What URLs Thunderbird queries (autoconfig, ISPDB, MX/SRV)
    * Why SMTP is being swapped back to `smtp.<domain>`
  * I can share logs if told exactly what to enable.

  * Given DNS already points autoconfig/autodiscover to Fastmail, is it better to host our own `mail/config-v1.1.xml` at `autoconfig.<domain>` instead of CNAME, to stop fallback loops?

Thunderbird won’t auto-configure Fastmail for my domain (stuck in autodiscovery loop / overwrites SMTP) **Summary:** After moving our domain from Google Workspace to Fastmail, Thunderbird still tries old/incorrect settings when adding accounts. One mailbox adds, others fail. I need true **auto-config** for all users (no manual server entry). --- Environment * **Client:** Thunderbird for macOS (current stable, 2025 UI) * **OS:** macOS (Apple Silicon) * **Mail provider (now):** Fastmail * **Domain:** custom domain (previously hosted on Google Workspace, now on Fastmail) --- ## What changed * Domain email hosting migrated from Google Workspace → Fastmail. * DNS updated: MX/SPF/DKIM/DMARC all pointing to Fastmail. * Added autoconfig/autodiscover CNAMEs to Fastmail. **Current DNS of note** ``` MX: 10 in1-smtp.messagingengine.com 20 in2-smtp.messagingengine.com CNAME: autoconfig.<domain> → autoconfig.fastmail.com autodiscover.<domain> → autoconfig.fastmail.com SPF (TXT): v=spf1 include:spf.messagingengine.com ?all DKIM: fm1/fm2/fm3 selectors pointing to *.dkim.fmhosted.com DMARC: v=DMARC1; p=reject; adkim=r; aspf=r; rua=... ``` Testing autoconfig directly in browser (e.g. `https://autoconfig.fastmail.com/mail/config-v1.1.xml?emailaddress=user@domain`) returns Fastmail’s correct IMAP/SMTP XML. --- ## The problem (repeatable) 1. In **Account Hub**, enter any address at the domain. 2. The dialog sits on **“Looking up configuration”** (spinning). Sometimes it shows **Configure manually**. 3. If reaching Outgoing settings and pressing **Test**, Thunderbird **overwrites** the SMTP I typed (`smtp.fastmail.com`) with **`smtp.<domain>`** (wrong) and then fails at the end. 4. For some accounts, Thunderbird shows: ``` Authentication error. Component returned failure code: 0x80550014 [nsIMsgIncomingServer.verifyLogon] ``` 5. One mailbox **does** add successfully. Additional mailboxes (real Fastmail mailboxes with their own App Passwords) **do not**, even though IMAP/SMTP login works in other clients. **Why manual config doesn’t solve it:** I need a repeatable **auto-config** experience for multiple staff machines and future hires. Manually entering server details isn’t viable, and Thunderbird’s “Test” step keeps overriding anyway. --- ## What I’ve already tried * Verified DNS (see above) and confirmed propagation. * Confirmed each user is a **real Fastmail mailbox** (not alias), IMAP enabled, App Password generated. * In Thunderbird: * Cleared Saved Passwords. * Removed/re-added accounts. * Used Advanced Config to hard-set `imap.fastmail.com:993 SSL/TLS` and `smtp.fastmail.com:465 SSL/TLS`. Thunderbird still overwrites SMTP on test. * Nuked local state: * Deleted `~/Library/Thunderbird/` (profiles, installs.ini, profiles.ini). * Cleared Keychain items for imap/smtp/domain. * Flushed macOS DNS cache. * After reset, one mailbox adds via auto-config, but others still fail or get SMTP swapped. --- ## What I’m asking the community for 1. **How to force Thunderbird to always use the domain autoconfig (CNAME to Fastmail) and never overwrite with its own probe?** * Is there an about\:config pref to disable the probe/replace behavior during SMTP test? * Can Thunderbird be told to prioritize domain autoconfig and skip ISPDB/MX guessing entirely? 2. **How to fully clear Thunderbird’s cached provider/domain configs.** * Beyond deleting `~/Library/Thunderbird` and Keychain, are there other places Thunderbird stores provider data? 3. **Logging/diagnostics.** * Which logging prefs/console settings show: * What URLs Thunderbird queries (autoconfig, ISPDB, MX/SRV) * Why SMTP is being swapped back to `smtp.<domain>` * I can share logs if told exactly what to enable. 4. **Best-practice for Fastmail domains in Thunderbird.** * Given DNS already points autoconfig/autodiscover to Fastmail, is it better to host our own `mail/config-v1.1.xml` at `autoconfig.<domain>` instead of CNAME, to stop fallback loops? --- ## Goal / Constraints * Need a **zero-touch, auto-config** experience for all domain users (each with their own Fastmail mailbox + App Password). * No manual server typing. * No SMTP being silently replaced during “Test.” * Looking for a definitive way to make Thunderbird trust/use domain autoconfig every time. Thanks for any guidance, prefs to flip, or diagnostic steps to pinpoint why Thunderbird keeps reverting SMTP and why only the first mailbox adds cleanly.